[concepts.equality] Fix grammar for P2102R0

Author: opensdh

source/concepts.tex

@@ -84,7 +84,7 @@
 required to not be modified.
 For the purposes of this subclause,
 the cv-qualification and value category of each operand
-is determined by assuming
+are determined by assuming
 that each template type parameter
 denotes a cv-unqualified complete non-array object type.